A universal definition of the Kondo energy from the orthogonality catastrophe

Abstract

The definitions of the Kondo energy in the numerical renormalization group (NRG) and the Friedel artificially inserted resonance (FAIR) theory fail sadly for small samples where their predicted Kondo energy increases, while in reality the Kondo effect disappears. Therefore a different, universal definition of the Kondo energy is proposed, which uses the evasion of the orthogonality catastrophe by the Kondo impurity. In the absense of the Kondo effect the multi-electron scalar product (MESP) between all occupied spin-up and spin-down states approaches zero (the so-called orthogonality catastrophe). In contrast in the Kondo ground state the corresponding conduction electrons of opposite spin are pairwise aligned within the Kondo energy. In the present paper the MESP is investigated for the FAIR solution of the Friedel-Anderson impurity. The MESP is numerically determined for the (enforced) magnetic and the singlet states as a function of the number N of Wilson states. The magnetic states show an exponentially decreasing MESP as a function of N. In the singlet state the ground state requires a finite MESP to optimize its energy. As a consequence there is no orthogonality catastrophe. Within the energy range of the Kondo energy the scalar product between corresponding (single electron) spin-up and spin-down states is very close to 1.000 and falls off beyond the Kondo energy. The energy which separates the two regions is well suited as a universal definition of the Kondo energy.